242 



NATURE 



[July 15, 1897 



stitute diflficulties in the study of ancient volcanic action. 

 But they have not proved insuperable. Such has been 

 the progress of this study, that it has already thrown 

 light on the structure and mechanism of the active 

 volcanoes of the present day. Thus a comparison of 

 the pas with the present may enable us to arrive at some 

 adequate conception of the nature and history of vol- 

 canoes in the geological history of the globe. 



" In this research," as Sir Archibald Geikie remarks, 

 "it is obvious that the presently active volcano must be 

 the basis and starting-point of inquiry. At that channel 

 of communication between the unknown inside and the 

 familiar outside of our globe, we can watch what takes 

 placean times of quiescence or of activity. We can there 

 study each successive phase of an eruption, measure 

 temperatures, photograph passing phenomena, collect 

 gases and vapours, register the fall of ashes or the flow 

 of lavas, and gather a vast body of facts regarding the 

 materials that are ejected from the interior, and the 

 manner of their emission. 



" Indispensable as this information is for the com- 

 prehension of volcanic action, it obviously affords after 

 all but a superficial glimpse of that action. We cannot 

 see beyond the bottom of the crater. We cannot tell 

 anything about the subterranean ducts, or how the 

 molten and fragmental materials behave in them. All 

 the underground mechanism of volcanoes is necessarily 

 hidden from our eyes. But much of this concealed 

 structure has been revealed in the case of ancient 

 volcanic masses, which have been buried and afterwards 

 upraised and laid bare by denudation. 



" In yet another important aspect modern volcanoes 

 do not permit us to obtain full knowledge of the subject. 

 The terrestrial vents, from which we derive our informa- 

 tion, by no means represent all the existmg points of 

 direct connection between the interior and the exterior 

 of the planet. We know that some volcanic eruptions 

 occur under the sea, and doubtless vast numbers more 

 take place there of which we know nothing. But the 

 conditions under which these submarine discharges are 

 effected, the behaviour of the outflowing lava under a 

 body of oceanic water, and the part played by frag- 

 mentary materials in the explosions, can only be sur- 

 mised. Now and then a submarine volcano pushes its 

 summit above the sea-level, and allows its operations to 

 be seen, but in so doing it becomes practically a terres- 

 trial volcano, and the peculiar submarine phenomena 

 are still effectually concealed from observation. 



" The volcanic records of former geological periods, 

 however, are in large measure those of eruptions under 

 the sea. In studying them we are permitted, as it were, 

 to explore the sea-bottom. We can trace how sheets of 

 coral and groves of crinoids were buried under showers 

 of ashes and stones, and how the ooze and silt of the 

 sea-floor were overspread with streams of lava. We are 

 thus, in some degree, enabled to realise what must now 

 happen over many parts of the bed of the existing ocean. 



" The geologist who undertakes an investigation into 

 the history of volcanic action within the area of the 

 British Isles during past time, with a view to the better 

 comprehension of this department of terrestrial physics, 

 finds himself in a situation of peculiar advantage. 

 Probably no region on the face of the globe is better 

 fitted than these islands to furnish a large and varied 

 body of evidence regarding the progress of volcanic 

 energy in former ages " (vol. i. pp. 5-6). 



Towards the close of his second volume, after adducing 

 in full detail the volcanic records of his native country, 

 the author remarks that 



"A review of the geological history of Britain cannot 

 but impress the geologist with a conviction of the 

 essential uniformity of volcanism in its manifestations 



NO. 1446, VOL. 56] 



since the early beginnings of geological time. The 

 composition and structure of the materials erupted from 

 the interior have remainedwith but little change. The 

 manner in which these materials have been discharged 

 has likewise persisted from the remotest periods. The 

 three modern types of Vesuvian cones, Puys and fissure- 

 eruptions, can be seen to have played their parts in the 

 past as they do to-day." 



After an introductory series of chapters dealing with 

 general principles of investigation and interpretation, 

 the work enters upon a detailed description of the 

 volcanic phenomena of the successive geological periods, 

 beginning with the most ancient. 



"Among the earliest igneous masses of which the rela- 

 tive geological date can be fixed are the dykes which 

 form so striking a system among the Archaean rocks of 

 the north-west of Scotland, and show how far back the 

 modern type of volcanic fissures and dykes can be traced. 

 No relic, indeed, has survived of any lavas that may have 

 flowed out from these ancient fissures, but so far as 

 regards underground structure, the type is essentially the 

 same as that of the Tertiary and modern Icelandic lava- 

 fields" (vol. ii. pp. 470-471). 



The early Palajozoic volcanoes formed cones of lava 

 and tuff comparable to those of such vents as Vesuvius 

 and Etna. As illustrations of the Vesuvian type in the 

 volcanic history of Britain, the author refers to the great 

 Lower Silurian Volcanoes of Cader Idris,Arenig, Snowdon 

 and the Lake District, and to the Old Red Sandstone 

 volcanoes of Central Scotland. In the Lake District the 

 pile of material ejected during Lower Silurian time was at 

 least 8000 or 9000 feet thick. In the Old Red Sandstone 

 basins of Central Scotland there were more than one 

 mass of lavas and tuffs thicker than those of Vesuvius. 



The Carboniferous volcanoes were not only abundant 

 and persistent in Scotland, but they attained there a 

 variety and development which give their remains an 

 altogether exceptional interest in the study of volcanic 

 geology. They are referable, from their characters and 

 their age, to two different types. Plateaux and Puys 

 (vol. i. p. 364). 



In the Plateau-type^ the volcanic materials were dis- 

 charged over wide tracts of country, so that they now 

 form broad tablelands or ranges of hills, reaching some- 

 times an extent of many hundreds of square miles and a 

 thickness of more than 1000 feet. Plateaux of this 

 character occur within the British area only in Scotland 

 where they are the predominant phase of volcanic inter- 

 calations in the Carboniferous system, and are eminently 

 characteristic of the earliest portion of that period. This 

 Plateau or Fissure-type is, among modern volcanoes, best 

 developed in Iceland. In that island, during a volcanic 

 eruption, the ground is rent open by long parallel fissures, 

 only a few feet or yards in width, but traceable sometimes 

 for many miles, and descending to an unknown depth 

 into the interior. From these fissures lava issues — in 

 some cases flowing out tranquilly in broad streams from 

 either side, in other cases issuing with the discharge of 

 slags and blocks of lava which are piled up into small 

 cones set closely together along the line of the rent. By 

 successive discharges of lava from fissures, or from vents 

 opening on lines of fissure, wide plains may be covered 

 with a floor of rock made up of horizontal beds. 



The author shows that, after the beginning of the 

 Carboniferous Limestone period, when eruptions of the 



